Key Idea: The amount of thermal energy an object has depends on the disordered motions of its atoms or molecules and the number and types of atoms or molecules of which the object is made.
Students should know that:
- The thermal energy of an object is related to the average speed of its atoms or molecules due to their random motion, the number of atoms or molecules, and the types of atoms or molecules of which the object is made. The amount of thermal energy an object has is related to these three factors alone, e.g. thermal energy is not due to atoms or molecules rubbing against one another.
- All matter has thermal energy because all matter is made up of atoms and molecules that are constantly in random motion. The thermal energy of an object is the sum of the kinetic energy (motion energy) of all of the individual atoms and molecules that make up the object.
- For objects that are made of the same number and type of molecules, the object that is made up of the molecules with the highest average speed (and therefore, the highest kinetic energy) will have the most thermal energy and the object that is made up of the molecules with the lowest average speed (and therefore, the lowest kinetic energy) will have the least thermal energy.
- With all else equal, the greater the number of atoms and molecules the greater the thermal energy, and the fewer the number of atoms and molecules the lower the thermal energy.
Boundaries:
- In assessment items, the temperature changes are limited to those that do not involve changes of state.
- Assessment items do not ask students to use formulas, such as 3/2 kT and m(ΔT)c, to calculate thermal energy. The sub-ideas above describe semi-quantitative relationships. Students are expected to compare relative speeds or amounts of kinetic energy of the atoms that make up the objects and the relative number of atoms that make up the objects to determine relative amounts of thermal energy.
- Items do not assess students’ knowledge of which kinds of atoms/molecules have more thermal energy.
- Items do not assess students’ knowledge of why thermal energy is related to the types of atoms or molecules an object is made of. They will only assess the idea that thermal energy is related to the types of atoms or molecules of that object.
- Items do not assess students’ knowledge of internal energy or the potential energy that exists between the atoms or molecules of a substance.
- Items do not assess the idea that absolute zero is the temperature a substance would have if all atomic and molecular motion were to stop.
Note: The speed and kinetic energy referred to in this idea are the speed and kinetic energy related to the random motion of the atoms and molecules that make up the object. (We consider the thermal energy of a stationary object to be identical to the thermal energy of the same object when it is moving in relation to the surface of the earth.)
Note: The term “heat” can be used in everyday conversation as a verb or a noun. When heat is used as a verb, the meaning is basically to raise the temperature of an object as in “I heated the water.” When heat is used as a noun, it usually is intended to mean “thermal energy,” but that use is not scientifically correct. Technically, “heat” is the energy transferred from one system to another (or between a system and its environment) due to a temperature difference between the systems (or between the system and its environment). Students are not expected to know the proper use of the term heat. To avoid confusion, the everyday use of “heat” as a noun will be avoided in assessment items for middle school and beyond. The use of “heat” as a verb does not cause a problem, however.
| Item ID Number |
Knowledge Being Assessed | Grades 4–5 |
Grades 6–8 |
Grades 9–12 |
Select This Item for My Item Bank |
|---|---|---|---|---|---|
N/A | 55% |
65% |
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||
N/A | 53% |
65% |
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||
N/A | 47% |
54% |
|
||
N/A | 42% |
60% |
|
||
N/A | 46% |
56% |
|
||
When the kinetic energy of molecules increases, the thermal energy of an object increases. | N/A | 42% |
50% |
|
|
N/A | 41% |
47% |
|
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N/A | 38% |
47% |
|
||
N/A | 37% |
51% |
|
||
The thermal energy of an object depends on the speed and type of molecules that make up the object. | N/A | 39% |
46% |
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|
N/A | 43% |
38% |
|
||
The thermal energy of an object depends on the number and type of molecules that make up the object. | N/A | 36% |
43% |
|
|
N/A | 35% |
44% |
|
||
N/A | 33% |
42% |
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||
N/A | 29% |
39% |
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||
N/A | 32% |
34% |
|
Misconception |
Student Misconception |
Grades 4–5 |
Grades |
Grades |
|---|---|---|---|---|
N/A | 46% |
46% | ||
N/A | 39% |
37% | ||
N/A | 28% |
21% | ||
N/A | 23% |
20% | ||
N/A | 20% |
16% | ||
When comparing two objects at the same temperature the smaller object has more thermal energy. | N/A | 17% |
17% | |
N/A | 26% |
19% | ||
N/A | 27% |
28% | ||
Cold/frozen objects do not have any thermal energy (AAAS Project 2061, n.d.). | N/A | 16% |
14% | |
Inanimate objects do not have any thermal energy (Herrmann-Abell & DeBoer, 2010). | N/A | 22% |
18% | |
Only things that are warm or hot have thermal energy (Herrmann-Abell & DeBoer, 2010). | N/A | 21% |
18% |
Frequency of selecting a misconception was calculated by dividing the total number of times a misconception was chosen by the number of times it could have been chosen, averaged over the number of students answering the questions within this particular idea.